The sharp increase in the network capacity in recent years has required optical transmission systems to have a larger capacity. Recently, there has been developed at a rapid rate a wavelength division multiplex (WDM) optical transmission system of 40 Gbps which is capable of higher-capacity transmission than the WDM transmission system of 10 Gbps that has been in practical use at present.
The optical transmission system of 40 Gbps not only suffers the waveform degradation caused by wavelength dispersion which has been problematic so far, but also is highly susceptible to a waveform degradation caused by polarization dispersion, resulting in greatly limited distances over which optical signals are transmitted.
The polarization dispersion is a dispersion caused by the different propagation delay times of polarized components of optical pulses (two optical modes including a TE mode and a TM mode) because of the deviation of a slightly elliptical optical fiber from a true circle and stresses applied to the optical fiber. Generally, the polarization dispersion is greater as the optical signal is transmitted at a higher rate and over a longer distance.
Old optical fibers which have been laid out so far (mainly outside Japan) include optical fibers having a large polarization dispersion in excess of 1 ps/km1/2. If an optical signal is transmitted through such an optical fiber over 100 km, for example, then a differential group delay (DGD) ΔT caused in the two polarized components is of about 10 ps which is ⅖ of one time slot of 25 ps of the optical signal that has a rate of 40 Gbps. Therefore, the optical signal has its waveform greatly degraded and is transmitted over a largely limited distance.
The polarization dispersion varies with time due to changes in the temperature and changes in the transmission path environment that are caused by stresses such as fiber touch (stresses developed in the optical fiber by a touch of the optical fiber by hand and winds applied to the optical fiber).
Fluctuations in the polarization dispersion caused by temperature changes are disclosed in Non-patent document 1, for example. According to Non-patent document 1, a correlation has been pointed out between varied amounts of polarization dispersion and ambient temperature changes of a single-mode fiber in an underground conduit over a distance of 48.8 km. The results shown indicate that temperature changes cause the polarization dispersion to vary at a relatively low rate in the order of minutes.
According to Non-patent document 2, the frequency of polarization dispersion fluctuations due to mechanical vibration has been reported. It has been reported in Non-patent document 2 that polarization dispersion fluctuations generally have a rate of kHz, i.e., several msec. In order to compensate for polarization dispersion fluctuations with high accuracy, it is therefore necessary not only to automatically compensate for polarization dispersion fluctuations depending on the state of the transmission path at the time of a system startup (non-adjustment, adjustment-free), but also to dynamically monitor the effect of a polarization dispersion and compensate for polarization dispersion fluctuations while following the polarization dispersion at a high rate in the order of msecs. while the system is in operation.
Furthermore, the actual environment is affected by a wavelength dispersion which has been greatly responsible for waveform degradations in the optical transmission systems available so far, in addition to the polarization dispersion described above. Consequently, the waveform of an optical signal that is received in the actual environment is complexly degraded by a combination of plural dispersion factors that exist at the same time. Even if attempts are made to compensate for complex waveform degradations caused by such plural dispersion factors with an existing wavelength dispersion compensator or an equalizer for compensating for general waveform degradations, no convergent solution can be obtained, failing to optimally compensate for the waveform degradations.
For making it possible to transmit optical signals over a distance of several hundreds km with an optical transmission system of 40 Gbps, the following two schemes are required:
(1) a scheme for automatically compensating for polarization dispersion at a rate in the order of msecs., and
(2) a scheme for optimally compensating for both waveform degradations caused by polarization dispersion and waveform degradations caused by wavelength dispersion while polarization dispersion and wavelength dispersion are occurring simultaneously.
A technology for automatically compensating for wavelength dispersion and polarization dispersion is disclosed in Patent document 1, for example. Specifically, an automatic dispersion compensating system disclosed in Patent document 1 includes a dispersion compensator for automatically compensating for wavelength dispersion of an optical signal transmitted through an optical transmission path, and a dispersion compensator for automatically compensating for polarization dispersion of the optical signal transmitted through the optical transmission path. Each of the dispersion compensators comprises a variable dispersion compensating device, a control monitor circuit, and a control circuit.
Patent document 2 discloses a technology for optimally controlling an amount of compensation for waveform degradations based on information about a code error of an optical signal. Specifically, in an automatic dispersion compensating system disclosed in Patent document 2, as shown in FIG. 1, an optical signal is amplified by optical amplifier 101 and thereafter sent to variable dispersion compensator 102. An output signal from variable dispersion compensator 102 is amplified by optical amplifier 103 and thereafter converted into an electric signal by photodetector 104. An output signal from photodetector 104 is amplified by amplifying circuit 105, subjected to a clock reproducing and identifying process by clock reproducing and identifying circuit 106, and subjected to series-to-parallel conversion by series-to-parallel converting circuit 107. Error correcting circuit 108 performs a code process on an output signal from series-to-parallel converting circuit 107 to detect a code error, and sends information about the detected code error to control circuit 109 through a feedback loop. Control circuit 109 optimally controls variable dispersion compensator 102 based on the information sent from error correcting circuit 108.
Patent document 3 discloses a technology for separating two dispersions that are primarily responsible for transmission quality degradations, i.e., wavelength dispersion and polarization dispersion, using a general transmission quality monitor instead of a wavelength dispersion monitor. Examples of transmission quality monitor include an error measurer, a transmission quality monitor for measuring a Q value, etc. An automatic dispersion compensating system disclosed in Patent document 3 includes, as shown in FIG. 2, optical receiver 110 comprising photodiode (PD) 111 for converting an optical signal into an electric signal, equalizing amplifier 114 for amplifying an electric signal and shaping the waveform thereof, equalizing amplifier 114 comprising preamplifier 112 and amplifier 113, clock extracting circuit 115 for extracting a clock signal from an output signal from equalizing amplifier 114, and identifying circuit 116 for identifying the state of the output signal from equalizing amplifier 114. Status monitor 117 monitors the statuses of PD 111, equalizing amplifier 114, and identifying circuit 116. Another automatic dispersion compensating system disclosed in Patent document 3 includes, as shown in FIG. 3, includes PMD compensator 118 that is controlled depending on a monitored result from PMD monitor 119 for compensating for PMD of a received optical signal that is input to dispersion compensator 120. Dispersion compensator 52 is controlled depending on a monitored result from dispersion monitor 122 which monitors optoelectric transducer (O/E) 121.
Specific technologies for wavelength dispersion compensation are disclosed in Patent documents 4 through 6 and Non-patent documents 3, 4, for example. Variable wavelength dispersion compensating devices for wavelength dispersion compensation include optical devices using a VIPA (Virtually Imaged Phased Array) disclosed in Non-patent document 5 and an FBG (Fiber Bragg Grating) disclosed in Non-patent document 6.
Specific technologies for polarization dispersion compensation include optically processing, optoelectrically processing, and electrically processing systems disclosed in Non-patent document 7, for example. The optically processing and optoelectrically processing systems require a polarized state to be controlled. Another control system for polarization dispersion compensation monitors a ½ frequency component and a ¼ frequency component of a clock signal included in an optical signal which has been compensated for polarization dispersion, and controls a polarized state, as disclosed in Non-patent documents 8, 9.
However, the dispersion compensating technologies according to the background art as described above have some problems. The problems of the respective dispersion compensating technologies will be described below.
(1) Specific technologies for polarization dispersion compensation include some technologies for performing optical processing and optoelectrical processing sequences. Most of those polarization dispersion compensating technologies need the polarized state to be controlled. However, it is difficult to realize polarization control on polarization dispersion that varies with time at a high rate in the order of msecs.
(2) The technology disclosed in Patent document 1 does not separate a control process for compensating for wavelength dispersion and a control process for compensating for polarization dispersion, and does not make the control processes operable together. A process required to make the control processes operable together tends to make the apparatus larger in size and higher in cost.
(3) The technology disclosed in Patent document 2 is effective to make the apparatus smaller in size and lower in cost because the dispersion identifying process and the control process are electrically performed. However, if dispersion fluctuations are developed owing to certain conditional changes, causing a code error, then the technology does not provide data for determining whether the dispersion is to be compensated for excessively (positively) or decrementally (negatively) from the present level. Generally, a method of finding an optimum compensating point is adopted by using an algorithm such as a dithering process for expressing gradations with randomly generated dots, as disclosed in Patent document 7, or a hill-climbing process for determining, as a next search course, one of routes which is likely to be closest to the goal, when next candidate vertexes are developed from the present vertex. However, the technology disclosed in Patent document 2 fails to initially determine whether the error rate decreases or increases when the amount of dispersion compensation increases, because it does not provide the above data. Specifically, when the amount of dispersion compensation is controlled based on the error rate, a long time is required until an optimum value is reached. Furthermore, if the amount of dispersion compensation is controlled at coarse intervals, then a convergent point for the system may possibly be not found. Consequently, the intervals at which the amount of dispersion compensation is controlled have to be highly accurate to a certain degree. With the highly accurate control intervals, the time required until the system converges and the number of repetitive cycles are increased. It is thus difficult for the technology to be applied to a system which needs to compensate for the dispersion at a high rate in the order of msecs.
(4) Of the technologies disclosed in Patent document 3, the automatic dispersion compensating system shown in FIG. 2 is a general automatic dispersion compensating system for extracting a control from an ordinary waveform monitor, and has no specific algorithm and configuration reviewed as to how to specifically separate the cause of wavelength dispersion and the cause of polarization dispersion. The automatic dispersion compensating system shown in FIG. 3 requires a monitor, a control device, and a dispersion compensator for each of wavelength dispersion and polarization dispersion, in the same manner as described with respect to (1). Since the dispersion compensator and its peripherals are mostly made up of optical components, the apparatus tends to be larger in size and higher in cost due to an increased number of components, and hence is less liable to be made versatile.
Patent document 1: JP-A No. 7-221705
Patent document 2: JP-A No. 2002-208892
Patent document 3: JP-A No. 2004-7150
Patent document 4: JP-A No. 8-321805
Patent document 5: JP-A No. 9-326755
Patent document 6: JP-A No. 10-276172
Patent document 7: JP-A No. 2002-33701
Non-patent document 1: “J. Cameron et al.: Time evolution of polarization—mode dispersion for aerial and buried cables, Proc. OFC98, pp 240-241”
Non-patent document 2: “H. Brow et al.: Measurement of the Maximum Speed of PMD Fluctuations in Installed Field Fiber, Proc. OFC'99, pp 83-85”
Non-patent document 3: “G. Ishikawa et al., “DEMONSTRATION OF AUTOMATIC DISPERSION EQUALIZATION IN 40 Gbits/s OTDM TRANSMISSION”, ECOC '98, pp. 519-520”
Non-patent document 4: “Y. Akiyama et al., “AUTOMATIC DISPERSION EQUALIZATION IN 40 Gbits/s TRANSMISSION BY SEAMLESS-SWITCHING BETWEEN MULTIPLE SIGNAL WAVELENGTHS”, ECOC '99, pp. I-150-151”
Non-patent document 5: “M. Shirasaki et al., “Dispersion Compensation Using The Virtually Imaged Phased Array”, APCC/OECC '99, pp. 1367-1370, 1999”
Non-patent document 6: “M. M. Ohn et al., “Tunable Fiber Grating Dispersion Using a Piezoelectric Stack”, OFC'97 WJ3”
Non-patent document 7: “H. Bulow et al., “Optical and electric PMD compensation”, OFC'03, p. 541”
Non-patent document 8: “H. Ooi et al., “Automatic Polarization—Mode Dispersion Compensation in 40 Gbits/s Transmission”, IOOC'99, WE5”
Non-patent document 9: “D. Sandel et al., “Automatic polarization mode dispersion compensation in 40 Gbits/s optical transmission system”, Electron. Lett., 1998, pp 2258-2259”
Non-patent document 10: “Makoto Nagaoka, Iwanami Lecture Series Software Science 14 “Knowledge and Deduction”, Iwanami Shoten, 1988, pp. 114-120”